Chemistry Reference
In-Depth Information
enzymes are protective, they produce H
2
O
2
. Since H
2
O
2
inhibits SOD, is the
precursor of HO
·
radicals and activation of hemeproteins to oxoferryl radicals,
H
2
O
2
molecules must be rapidly eliminated from tissues. In animal cells, two
enzymes exist to rapidly remove H
2
O
2
: catalase and glutathione peroxidase. In
plants, there are catalase and several peroxidases including ascorbate
peroxidase. Catalase by one cycle degradates two H
2
O
2
molecules to water
and oxygen, but peroxidases by one cycle degrades only one molecule of H
2
O
2
(Halliwell and Gutteridge, 2007).
Catalase±Fe
3
H
2
O
2
ÿ! Catalase±Fe
4
=O H
2
O
2.34
Catalase±Fe
4
=O H
2
O
2
ÿ! Catalase±Fe
3
H
2
O O
2
2.35
Glutathione peroxidases are enzymes which reduce H
2
O
2
to water and lipid
hydroperoxides to alcohols. There are several isozymes such as glutathione
peroxidase (GPX1), which is found in cytoplasm whose preferred substrate is
H
2
O
2
. GPX4 has a high preference for lipid hydroperoxides, GPX2 is an
intenstinal and extracellular enzyme, while GPX3 is extracellular, especially
abounded in plasma. So far, eight different isoforms of glutathione peroxidase
have been identified (Brigelius-Flohe, 1999; Halliwell and Gutteridge, 2007).
Glutathione peroxidases have been shown to be with a selenium-cysteine active
center acting by the following reactions:
GPX
2GSH H
2
O
2
ÿÿÿ!
2GS±SG 2H
2
O
2.36
GPX
2GSH LOOH ÿÿÿ!
2GS±SG LOH H
2
O
2.37
where GSH represents reduced glutathione and GS-SG glutathione disulfide,
LOOH lipid hydroperoxide and LOH, hydroxy-fatty acid. Glutathione reductase
then reduces the oxidized glutathione to complete the cycle.
2.5 Metal catalyzed oxidation in beverages
Citrus juices, especially orange and grapefruit, are popular breakfast beverages
and a good source of vitamin C and folic acid in human nutrition. The juices also
contain flavonoids that are believed to have beneficial health effects. However,
all those vitamins and flavonoids are sensitive to heat-treated process and
storage temperatures, due to aerobic and anaerobic reactions of non-enzymic
nature.
Ascorbic acid destruction rates were directly proportional to initial concen-
tration of dissolved oxygen and presence of transition metals in model systems
and products (Kanner and Shapira, 1989). Storage studies on the loss of ascorbic
acid potency in canned orange juices have shown an initial period of rapid loss
of ascorbic acid that was caused by the presence of free oxygen from the head
space or this penetrate through packaging material.
The involvement of ascorbic acid in non-enzymic browning of citrus products
is well known (Lee and Nagy, 1988; Kanner and Shapira, 1989; Johnson et al.,
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